Limit control for motors



May 11, 1965 F. A. MANNERS LIMIT CONTROL FOR MOTORS Filed Sept. 22. 1961 LIMIT SWITCH Ill mmvron M at", 721m United States Patent 3,183,423 LIMIT CONTROL FOR MOTORS Frank Alan Manners, Cleveland, Ohio, assignor to Square D Company, Park Ridge, IiL, a corporation of Michigan Filed Sept. 22, 1961, Ser. No. 140,053 Claims. (Cl. 3182tl3) This invention relates to a reversing control system for a polyphase induction motor and electroresponsive brake combination, and more particularly to such a control system having a power circuit limit switch operative to deenergize the motor and set the brake upon movement of a load driven by the motor beyond a predetermined limit in one direction.

Although having other applications, the invention will be described as embodied in a control system for a motor driving a hoisting mechanism of a crane. The substitution of saturable devices or other static switching means for the electromagnetic contactors previously used in alternating current crane hoist control systems has presented problems related to the interposition of the contacts of an overhoist limit switch in the supply lines to the motor primary and to a brake for the hoist mechanism. Such a limit switch must be so connected that when it is in its normal and untripped position, the motor can be operated in the hoisting and lowering directions, selectively. Upon tripping of the limit switch by reason of the hoist mechanism being driven too far in the hoisting direction, the motor must be disconnected from the power source and the brake must set automatically. It has been found desirable that the limit switch, upon tripping, complete auxiliary circuits making it possible to release the brake and cause the motor to rotate in the lowering direction in order to reset or return the limit switch to its normal position. Upon resetting of the limit switch, the motor and brake should be controllable in the same manner as they were prior to tripping of the limit switch.

The control system of the present invention, as hereinafter described, not only has the foregoing capabilities, but, in addition, insures safe operation of the motor in the lowering direction while the limit switch is tripped regardless of the overhauling or non-overhauling nature of the motor load and regardless of whether or not the limit switch resets promptly.

It is an object of this invention to provide an improved control system for a polyphase induction motor and electroresponsive brake combination including a limit switch interposed in the supply circuits for the motor and brake.

Another object is to provide a control system for an alternating current motor in which an electroresponsive brake for the motor is energized through a transformer controlled by a limit switch.

Another object is to provide a limit control system for an alternating current motor and electroresponsive brake combination in which the brake is connected for energization from a source of alternating current through a secondary winding of a transformer, a limit control means normally interconnects the terminals of a primary winding of the transformer across a circuit of low impedance thereby to render the secondary winding impedance low, said limit control means becoming operative, while said secondary winding impedance is low, upon the motor reaching a limit of operation, to interrupt said circuit of low impedance and to connect the brake to the source of power through the secondary winding whose induced voltage is in series opposition to said source or" alternating current.

Other objects and advantages of this invention will become apparent from the following specification wherein reference is made to the drawing, in which:

The drawing is a schematic wiring diagram of a control system embodying this invention.

3,183,423 Patented May 11, 1965 Referring to the drawing, the invention is disclosed for purposes of illustration as applied to a three-phase wound rotor induction motor 10, its use with squirrel cage and other types of motors being readily apparent from the i lustrative example. The motor 10 has a primary winding 19a provided with terminals T1, T2, and T3 and supplied from a suitable power source, such as supply conductors L1, L2, and L3. The motor has a secondary winding 10b connected to a balanced Y-connected network each leg of which includes a resistor 11 which, if desired, may be adjustable. The eiiective connection of the primary winding lltla to the power source L1, L2, and L3 and the phase rotation and magnitude of the voltages applied to the primary winding 10a is controlled by a plurality of saturable reactors having reactance windings 1H, 2H, 1L, 2L, and C. The impedances of the windings 1H and 2H are controlled by a control winding HX. The impedances of the windings 1L and 2L are controlled by a control winding LX. The impedance of the winding C is controlied by a control winding CX. It will be understood that other torque and direction control means may be used to perform the primary switching for the motor 10.

An overhoist limit switch 18 is arranged to be operated by a load 1? driven by the motor 10. The switch has two normally open contacts 18a and 18b and two normally closed contacts 18c and 13d. The contact 1811 is interposed between the motor terminal T2 and one end of a resistor 2% which has its other end connected to the terminal T1 and to the common load terminal of the windings 1H and TL. The contact 18b is interposed between the terminal T3 and the supply conductor L3. The contact is interposed between the terminal T2 and a load terminal of the winding C. The contact 18d is interposed between the terminal T3 and a common load terminal of the windings 2H and 2L.

The reactance windings are connected as follows: winding 1H directly between conductor L3 and terminal Tl; winding 2H between conductor L1 and terminal T3 through contact 18d; winding C between conductor L2 and terminal T2 through contact 18c; winding 1L directly between conductor L1 and terminal T1; and winding 2L between conductor L3 and terminal T3 through contact 13d.

A transformer 21, preferably of a one-to-one ratio, has a primary winding 21p connected directly across the contact 18d and connected through one of its terminals between the contact 1812 and terminal T3. A secondary winding 21s is connected between the supply conductor L3 and one of the input terminals of a full-wave rectifier 24. A spring-applied, electrornagnetically-released friction brake 25 having an operating winding 25w is provided for the motor 16. The output terminals of the rectifier 24 are connected to supply the winding 25w through a normally open contact 26a to an electromagnetic brake relay 26 which has a normally open contact 26b and an operating winding 26w. The contact 26b is interposed in a connection between the supply conductor L1 and the other input terminal of the rectifier 24. Although an electromagnetic brake is illustrated, it will be understood that other suitable types of electroresponsive brakes can be used.

The control windings HX and LX are supplied from controlled full-wave rectifiers 28 and 29, respectively, connected in parallel with each other across a pair of supply conductors 30 and 31 extending from terminals 34 of a suitable alternating current source (not shown). A fullwave rectifier 35 for the control winding CX is interposed in the conductor 31. Each of the rectifiers 28 and 29 has four rectifier units, two of which are controlled units. The controlled rectifier units of rectifier 28 are indicated at 36 and 37 and are supplied with respective control signal voltages from a hoisting control unit 38. The controlled rectifier units of rectifier 29 are indicated at 39 and 40 and are supplied with respective control signal voltages from a lowering control unit 41.

The control units 38 and 41 are normally turned off and are arranged to be selectively turned on in response to a differential signal voltage which is the resultant of a reference voltage and a feed-back voltage in series opposition. The reference voltage is obtained from a reversing master control element 42 supplied from the source 34 through a rectifier 44. The feedback voltage is obtained from a tachometer generator 45 driven by the motor 'so that the feed-back voltage is proportioned to the speed of the motor 10. When the hoisting control unit 38 is turned off, the rectifier units 36 and 37 are non-conducting, and when the control unit 38 is turned on, the rectifier units 36 and 37 conduct to a degree related to the magnitude of the differential signal voltage. Similarly, when the lowering control unit 41 is turned oh, the rectifier units 39 and 40 are non-conducting, and when the control unit 41 is turned on, the rectifier units 39 and 4t) conduct to a degree related to the magnitude of the differential signal voltage.

The master control element 42 includes a potentiometer resistor 46 connected across the direct current output terminals of the rectifier 44, a center tap 47 on the resistor 46, and a manually operable slider 48 movable along, and in electrical contact with, the resistor 46 and electrically engageable with the center tap 47. The polarity of the reference voltage from the master control element 42 is determined by the position of the slider 4% with respect to the center tap 47. The magnitude of the reference voltage is proportional to the distance of the slider 43 from the center tap 4'7 so that the polarity of the potential between a conductor 49, connected to the slider, and a conductor 50, connected to the center tap 47, can be selected and its magnitude adjusted or varied by movement of the slider 48.

The master control element 42 also includes a contact mechanism 51 including a contact 51a which is opened, as by a cam 51b, when the slider 48 is in its center position in engagement with the center tap 47 and is released by the cam 51b and closes Whenever the slider 48 is moved from its central position.

Assuming the motor 10 to be at rest and the brake 25 to be set, movement of the slider 48 of the master control element 42 in the clockwise direction from its central position provides a reference voltage of a polarity to turn on the lowering control unit 41. This movement of the slider 48 also causes closure of the conact 51a resulting in energization of the winding 26w of the relay 26 from the rectifier 44, thereby to effect release of the brake 25. Because the contact 18d of the limit switch is closed, the primary winding 21p is shunted and cannot support voltage. Therefore, the secondary winding 21s of the transformer 21 acts as a low impedance, allowing substantially the full voltage between the supply conductor L1 and L3 to be impressed on the rectifier 24 so that the winding 25w is energized and releases the brake 25.

Upon turning on the lowering control unit 41, the controlled rectifier 29 responds thereto to cause energization of the winding LX from the source 34, thereby lowering the impedance of the windings 1L and 2L. The current flowing through the winding LX also passes through the winding CX so that the impedance of the winding C is concurrently lowered. The reduction of the impedance of the windings 1L, 2L, and C causes a balanced polyph-ase voltage to be impressed on the motor terminals T1, T2, and T3 of such a phase rotation that the motor 10 exerts a torque in the lowering direction. Because the brake 25 is released, the load 19, whether overhauling or not, now starts to move downwardly and the tachometer generator 45 starts to generate a deed-back voltage.

As the speed of the motor 16) increases, the feed-back voltage from the tachometer generator 45 increases, causing the differential voltage to decrease, and thereby re sulting in a gradual increase in the impedance of the windings of 1L, 2L, and C. Should the feed-back voltage exceed the reference voltage from the master control element 42, as under an overhauling load condition, the polarity of the differential signal voltage reverses. Thereupon, the phase rotation of the voltage at the motor terminals T1, T2, and T 3 reverses because the lowering control unit 41 becomes turned off and concurrently the hoisting control unit 38 becomes turned on to cause a reduction in impedance of the windings 1H and 2H. This causes the motor 10 to exert a hoisting torque while still rotating in the lowering direction, thereby to provide a countertorque which maintains a lowering speed of the motor 16 determined by the distance of the slider 4% rom the center tap 47.

in event the load 19 is not overhauling, the phase rotation of the voltage at the terminals T1, T2, and T3 remains in such direction that the motor it) continues to exert a lowering torque to drive the load 19' downwardly at a speed determined by the position of the slider 48.

Upon return of the slider 48 to the central position at the tap 47, the contact 51a opens, the brake 25 sets, the reference voltage becomes zero, and the feed-back voltage is such that the impedances of the reactors 1H, 2H, and C are reduced to cause the motor to exert a hoisting torque, thereby to aid in stopping the motor quickly. The feed-back voltage is now zero and there is no voltage at the motor terminals Tl, T2, and T3.

Upon movement of the slider 48 in a counterclockwise direction from the central position, the brake 25 is released as before, and the reference voltage is of such polarity that the hoisting control unit 48 is turned on. The turning on of the unit 38 causes the controlled rectifier 2% to supply current to the windings HX and CX to partially saturate the windings 1H, 2H, and C, and thereby to cause the voltage at the terminals T1, T2, and T3 to be of such phase rotation as to cause the motor 10 to exert a hoisting torque. When the motor it) starts to rotate in the hoisting direction, the interaction of the reference and feed-back voltages causes the torque of the motor iii to be suilicient to hoist the load 19 at a speed determined by the position of the slider 48. The speed of the motor 10 stabilizes when the reference voltage from the master control element 42 and the feed-back voltage from the genera-tor 4-5 effectively combine to indicate to the rectifier control unit 3% that the motor is operating at the desired hoisting speed.

During the lowering and hoisting operations so far described, the limit switch 18 is in its normal untripped position, and the contact 18d effectively short circuits the primary winding 21p of the transformer 21 so that the secondary winding 21s is of low impedance. Thus, upon closure of the contact 26b, substantially the full voltage between the supply conductors L1 and L3 is impressed on the rectifier 24.

Upon tripping of the limit switch 18, the contact 13d opens to remove the efiective short circuit of the primary winding 21p. Concurrently, the contact 18b closes to connect the right-hand end of the winding 21p to the supply conductor L3. If the slider 48 of the master con.- trol element 42 is positioned for hoisting operations, the winding 2H has a low impedance, and the left-hand end of the winding 21p is effectively connected to the supply conductor Lil except for the low voltage drop acros the winding 2H. The voltage thus established across the primary winding 21p is reflected in the secondary Wind ing 21s, which, due to its then existing connections in the circuit, provides a voltage which effectively opposes the voltage of the supply conductors L1 and L3 impressed on the circuit including the series connected rectifier 24 and secondary winding 21s. The net voltage now irn-' pressed on the rectifier 24 is very low so that the voltage across the winding 25w is below its drop-out voltage, and the brake 25 sets.

Further, upon tripping of the limit switch 18, the terminal T3 is disconnected from the common terminal of the windings 2H and 2L by opening of the contact 18d, and is connected to the conductor L3 through the now closed contact 18b; the terminal T2 is disconnected from the winding C by opening of the contact 18 and is connected to the terminal Til through the resistor 20 and the now closed contact 18a. The voltage applied to the motor is reduced to a negligible value because the terminals T1 and T2 are effectively connected to supply conductor L3.

To operate the motor 19 in a lowering direction with the limit switch tripped, it is necessary for the brake to be released and for the motor to produce only sufficient driving down torque in order to overcome friction when lowering an empty hook. A load 19 of any size on the hook assists the motor in this operation. Upon reduction in the impedance of the windings 1L and 2L resulting from movement of the slider 48 into a lowering position, single-phase power is applied to the terminals T1 and T3 with the resistor 20 connected between terminals T1 and T2. The resistor 20 produces an unbalanced three-phase voltage at the terminals T1, T2, and T3 to cause the motor to develop adequate driving down torque. The transformer winding 21p is connected across winding 2L through the contact 18b. Because the winding 2L is now of low impedance, the winding 21s is of low impedance and sufiicient voltage appears at the rectifier 24 to release the brake 25 upon energization of the relay 26. Thus, it is only necessary to move the master control element 42 in the lowering direction to lower an empty hook or the load 19 out of the limit switch zone. When the limit switch 18 resets, the connections to the motor and to the brake rectifier 24 are the same as described above for lowering operation.

An additional safety feature prevents an overspeed condition when lowering out of the limit switch zone even if the limit switch 18 should fail to reset and remain tripped. If the load 19 is overhauling, the feed-back voltage from the tachometer generator 45 causes the differential voltage to increase the impedance of winding 2L so that the voltage impressed on the rectifier 24 is reduced to a value which will cause the brake 25 to set and stop the motor 10. When the motor stops, the feed-back voltage disappears, and the reference voltage causes the impedance of the Winding 2L to decrease again. The brake 25 is consequently released and the load 19 again accelerates and then stops as before. Therefore, if the limit switch 18 fails to reset, the motor 10 will alternately accelerate and stop, thus safely lowering the load 19 even if the operator holds the master switch 42 in a lowering position.

Having thus described my invention, I claim:

1. A limit control system for a polyphase motor and electroresponsive brake therefor, said system comprising a polyphase motor, a circuit connected to said motor and adapted for connection to a source of poly hase voltage, variable impedance means in said circuit for controlling the direction of phase rotation and the magnitude of the voltage applied to the motor from said source, an electromagnetic brake, a transformer having a primary and a secondary winding, means for connecting the brake for energization from a single phase of said source through said secondary winding, a limit control means normally connecting said primary winding across a circuit of low impedance thereby to render said secondary winding of low impedance, said limit control means being operative, upon said motor reaching a limit of operation in one direction While said secondary winding impedance is low, to interrupt said circuit of low impedance and to connect said primary winding to said source and to connect said secondary winding and said source in series opposition with each other.

2. The limit control system of claim 1 characterized in that said limit control means includes additional means operative upon said motor reaching said limit of operation to reduce the voltage at the motor to a negligible value and to unbalance the voltage at the motor so as to permit rotation of the motor in the opposite direction upon variation of said impedance means.

3. The limit control system of claim 2 wherein said additional means unbalances the voltage at the motor by interposing a resistor between said source and one motor terminal while connecting the other two terminals across a single phase of said source.

4. The limit control system of claim 1 characterized in that means are provided which are operative, after operation of said limit control means, to supply voltage of operating magnitude to said motor for causing operation of said motor in the opposite direction and to connect said primary winding in a second circuit of low impedance.

5. The limit control system of claim 1 wherein means are provided to render said limit control means operative for energization from a single phase of said source so that the voltage induced in said secondary Winding by current flow in the primary winding is in series opposition to the voltage impressed on said secondary winding directly from said source.

References Cited by the Examiner UNITED STATES PATENTS Re. 22,923 9/47 Wickerham 318-369 X 2,460,234 1/49 Myles et a1. 31823O X 2,597,136 5/52 Snyder 318-229 X ORIS L. RADER, Primary Examiner. JOHN F. COUCH, Examiner. 

1. A LIMIT CONTROL SYSTEM FOR A POLYPHASE MOTOR AND ELECTRORESPONSIVE BRAKE THEREFOR, SAID SYSTEM COMPRISING A POLYPHASE MOTOR, A CIRCUIT CONNECTED TO SAID MOTOR AND ADAPTED FOR CONNECTION TO A SOURCE OF POLYPHASE VOLTAGE, VARIABLE IMPEDANCE MEANS IN SAID CIRCUIT FOR CONTROLLING THE DIRECTION OF PHASE ROTATION AND THE MAGNITUDE OF THE VOLTAGE APPLIED TO THE MOTOR FROM SAID SOURCE, AN ELECTROMAGNETIC BRAKE, A TRANSFORMER HAVING A PRIMARY AND A SECONDARY WINDING, MEANS FOR CONNECTING THE BRAKE FOR ENERGIZATION FROM A SINGLE PHASE OF SAID SOURCE THROUGH SAID SECONDARY WINDING, A LIMIT CONTROL MEANS NORMALLY CONNECTING SAID PRIMARY WINDING ACROSS A CIRCUIT OF LOW IMPEDANCE THEREBY TO RENDER SAID SECONDARY WINDING OF LOW IMPEDANCE, SAID LIMIT CONTROL MEANS BEING OPERATIVE, UPON SAID MOTOR REACHING A LIMIT OPERATION IN ONE DIRECTION WHILE SAID SECONDARY WINDING IMPEDANCE IS LOW, TO INTERRUPT SAID CIRCUIT OF LOW IMPEDANCE AND TO CON- 